This invention generally relates to wiring boards, and more particularly is concerned with printed wiring assemblies PWAs with controlled thermal expansion characteristics.
In today's aviation industry, it is common for a single aircraft to be subjected to several extreme thermal conditions in a relatively short time interval. It is not uncommon for an aircraft to be flying at an altitude of 40,000 feet with an outside temperature of less than -40.degree. F., while only moments earlier it was waiting for a take-off clearance from a hot, humid airport runway. With the current aspirations for trans-atmospheric aircraft, these extreme vicissitudes in the ambient temperature will continue to confront avionics engineers with perplexing problems of increasing difficulty and importance.
One particular problem that is exacerbated by these temperature oscillations is the frequent failure of solder connections between the leads of surface mounted devices (SMD's) and the corresponding pads of printed wiring boards (PWB's). with such PWAs, temperature swings cause the solder joint between the SMD and the PWB to be subjected to a series of stresses. Typically, the PWBs are of a glass/epoxy laminate or other non-conductive material which has a different coefficient of thermal expansion from the SMDs, which are normally fabricated from ceramic materials. This difference in expansion coefficients results in differing degrees of expansion to occur and the intermediate solder joint to be stressed. This problem is increasingly prevalent in PWAs having large multi-leaded SMDs thereon. Larger SMDs typically have many leads which are very small in comparison to the overall size of the SMD. Furthermore, the lead sizes are not typically changed when the overall size of the SMD is increased; therefore, the ratio of the largest linear dimension of the SMD, which is typically directly related to the stress intensity upon the joint, compared with the lead size, increases whenever the overall size of the SMD increases. Ultimately, the series of differential expansion and contraction places sufficient cumulative stress on the intermediate solder joint to cause both mechanical and electrical failure to occur in large SMDs.
Several alternative methods have been used in attempts to extend the number of cycles before failure in temperature cycling. One method has been tried where the SMDs are attached to a conventional PWB, but in addition, a layer of copper-invar-copper (CIC) is inserted near the top and bottom surfaces with the SMDs being mounted on either side. CIC has a thermal coefficient of expansion which is almost equal to that of the SMD. This "brute force" approach actually limits the differential expansion that can occur, because the wiring board is physically bound to, and restricted from excess expansion by, the underlying CIC.
Frequently, the metal chosen is copper coated invar. When the copper coated invar is used as an expansion controlling metal it becomes necessary to drill a hole through the CIC in order to create the plated through holes which interconnect the several layers of a multi-layered board.
While this method has been used extensively in the past it does have several serious drawbacks. One major problem with utilizing copper-invar-copper inserted in a multi-layered wiring board and drilling holes to provide an interconnect between the several layers is that it is frequently difficult to achieve good plating to the invar surface. If faults in plating to the invar surface occur it may cause a failure in the wiring board performance. Another problem with such boards is that the exposed invar layer in the holes can act as a heat sink and thereby cause additional heating to be required when soldering takes place.
Consequently, there exists a need for a method to interconnect CIC within a multi-layered wiring board so that it has a high degree of reliability.